Transcoder and Method of Transcoding Therefore

ABSTRACT

A transcoder comprises a receiver ( 101 ) which receives input data representing an encoded signal and comprising first encoding data and first parametric extension data. The encoded data is fed to a decoder ( 103 ). The output of the decoder ( 103 ) is fed to an encoder ( 105 ) which generates second encoded data according to a different encoding protocol or with different encoding parameters. The first parametric extension data is fed to an extension data processor ( 109 ) which generates second parametric extension data directly from the first parametric extension data. The second encoded data and the second parametric extension data is combined in an output processor ( 107 ) to generate a transcoded signal comprising separately determined parametric extension data. The parametric extension data may be Spectral Band Replication (SBR) or Parametric Stereo (PS) extension data for an audio bitstream. Improved quality and reduced complexity is achieved by the separate transcoding of the parametric extension data.

FIELD OF THE INVENTION

The invention relates to a transcoder and method of transcodingtherefore and in particular to transcoding of audio signals.

BACKGROUND OF THE INVENTION

In recent years, the distribution and storage of A/V content in digitalform has increased substantially. Accordingly, a large number of codingstandards and protocols have been developed including for example MPEG-2audio and video coding.

One of the most widely known coding standards for digital coding ofaudio signals is the MPEG-1 Layer 3 standard, described in ISO/IECJTC1/SC29/WG11 MPEG, IS11172-3, Information Technology—Coding of MovingPictures and Associated Audio for Digital Storage Media at up to about1.5 Mbit/s, Part 3: Audio, MPEG-1, 1992, generally referred to as MP3.As an example, MP3 allows, a 30 or 40 megabyte digital PCM (Pulse CodeModulation) stereo audio recording of a song to be compressed into e.g.a 3 or 4 megabyte MP3 file. The exact compression rate depends on thedesired quality of the MP3 coded audio. Another example of an audiocoding standard is AAC (Advanced Audio Coding), described in ISO/IECJTC1/SC29/WG11 MPEG, IS13818-7, Information Technology—Generic Coding ofMoving Pictures and Associated Audio, Part 7:Advanced Audio Coding,1997.

Audio coding and compression techniques such as MP3 or AAC provide forvery bit-rate efficient audio coding which allows audio files ofrelatively low data size and high quality to be conveniently distributedthrough data networks including for example the Internet. However, moreefficient techniques that may reduce the bandwidth requirement orincrease the quality of the coded signals are desirable. For example,the increase in distribution of audio files over the Internet over thelast years has resulted in an accumulation of the network load.Furthermore, lower encoding data rates will further reduce the downloadtime.

Consequently, significant research has been undertaken to provide moreefficient coding techniques. However, due to the widespreaddissemination of existing coding techniques, it is preferable for newtechniques to be backwards compatible with one or more of these.

Two technologies which have recently been developed for encoding ofaudio signals are known as Spectral Band Replication (SBR) andParametric Stereo (PS) coding. These technologies can be applied on topof any audio coding scheme in a backwards compatible fashion.Specifically, SBR and PS generate enhancement data, which may be used toreduce the bit rate for encoding the audio signal in for example MP3 orAAC format. The enhancement data may be stored in ancillary datasections of the MP3 or AAC data stream thereby allowing conventionaldecoders to ignore the additional data.

In Parametric Stereo (PS), stereo audio encoding is achieved by encodingonly a single mono signal using e.g. MP3 or AAC. In addition, stereoimaging parameters are determined in the encoder and included in thedata stream as separate extension data. At the decoder, the mono encodedchannel is expanded into stereo channels by processing the mono encodedsignal differently for the two channels dependent on the stereo imagingparameters. These parameters consist of Inter-channel IntensityDifferences (IID), Inter-channel Time or Phase differences (ITD or IPD)and Inter-channel Cross-Correlations (ICC).

In a Spectral Band Replication (SBR) enhanced encoder, a low frequencyband of the audio signal to be encoded is extracted. This low frequencyband is subsequently encoded using a suitable encoding technique such ase.g. MP3 or AAC. In addition, the SBR encoder generates high frequencyparameters which are included in the data stream as enhancement data.Thus, the high frequency band of the audio signal is not encoded in thesame fashion as the low frequency band but is parametrically encoded.Specifically, the high band is created by a transposition of the lowfrequency band together with high frequency parameters which comprisedata indicating how the transposed signal should be processed (e.g. byenvelope modification) to generate the high frequency band. An SBRdecoder extracts the high frequency parameters and generates the highfrequency band by modifying the transposed low frequency band accordingto these high frequency parameters. Specifically the SBR high frequencyparameters include the following information:

-   -   Transposition information (i.e. information indicating the        mapping between low frequency band sub-bands and high frequency        band sub-bands).    -   Spectral envelope data The spectral envelope data indicates the        energy values of the sub-bands after SBR processing.    -   Noise floor data. The noise floor data together with the        estimated energy of the transposed signal (this estimate is        calculated in the SBR decoder) indicates the amount of noise        that is to be added to a high band signal.    -   Optionally, information on absent high frequency components        (e.g. harmonics which are present in high band, but not in the        low band).

An MP3 encoder with an SBR enhancement is known as an mp3PRO encoder andan AAC encoder with an SBR enhancement is known as an aacPlus or HighEfficiency (HE)-AAC encoder.

For both SBR and PS the enhancement parameters can be efficientlyencoded into the ancillary data portion of the core-coding scheme aslong as the data rate of the enhancement parameters does not exceed theavailable capacity of the ancillary data sections. Legacy decoders willnot process this ancillary data but will only decode the core-encodeddata. For SBR this is a band limited signal and for PS a full bandmonaural signal. In this way backwards compatibility is maintained asaudio signals, albeit at reduced quality, may be generated by legacydecoders.

Due to the variety of different coding standards and technologies, it isfrequently convenient to transcode between different coding standards ordifferent coding settings of the same coding standard. Thus, transcodingis used to convert a bit-stream of format A to the same format A withdifferent coding parameters (e.g. bit-rate, sampling rate) or to adifferent format B. Conventionally, a transcoder implements a cascade ofa decoder and an encoder such that the incoming signal is first decodedaccording to the format of the input data and subsequently re-encodedaccording to the format of the output data stream.

Generally, this will result in a quality loss. The issue of transcodingis further complicated when coding schemes are combined with parametricextensions such as SBR and/or PS. Since these extensions represent partsof the signal in a parameterized form, compared to representing thewaveform as faithfully as possible, larger quality degradations areexpected as a result of transcoding.

Furthermore, the complexity of the transcoding may increase due to theparametric extensions as the decoder must process the incoming extensiondata and the encoder must generate new extension data. This may resultin e.g. increased cost, computational requirement, delay etc.

Hence, an improved transcoding would be advantageous and in particular atranscoding providing improved performance, increased quality, reduceddata rate and/or reduced complexity would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention preferably seeks to mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination.

According to a first aspect of the invention, there is provided atranscoder comprising: means for receiving input data representing anencoded signal and comprising first parametric extension data; means fordetermining second parametric extension data from the first parametricextension data; and means for generating transcoded data including thesecond parametric extension data.

The inventors of the current invention have realized that parametricextension data for transcoded data may be directly generated fromparametric extension data of the input data. The invention mayaccordingly provide for an improved processing of parametric extensiondata in a transcoder without requiring that the parametric extensiondata is included in a decoding and re-encoding process. The inventionmay accordingly allow a reduced complexity of the transcoder.Alternatively or additionally, the transcoder may provide improvedquality of the transcoded data as parametric extension data of improvedquality may be determined, and as quality reduction associated with adecoding and re-encoding process may be mitigated or obviated.

The parametric extension data may comprise parameter data which may beused by a parametric decoder to enhance the quality of an encodedsignal. Parametric extension data may for audio coding representparameters according to an audio signal source model that describes thecomplete or a specific part of an audio signal.

For example, the first and/or second parametric extension data maycorrespond to extension data of e.g. a Spectral Band Replication (SBR)process and may for example include transposition information, spectralenvelope data and/or noise floor data. As another example, the firstand/or second parametric extension data may correspond to extension dataof e.g. a Parametric Stereo (PS) process and may for example includeInter-channel Intensity Differences (IID) data, Inter-channel Time orPhase differences (ITD or IPD) data and/or Inter-channelCross-Correlation (ICC) data. As a third example, the first and/orsecond parametric extension data may correspond to spatial multi-channelextension data. For example, the encoded signal may be a backwardscompatible stereo signal and the parametric extension data may comprisedata which allows generation of further spatial channels, such as forexample center and rear channels.

The input data may be an input data stream and the transcoded data maybe a transcoded data stream.

According to a feature of the invention, the input data furthercomprises first encoding data associated with the encoded signal and thetranscoder further comprises: means for transcoding the first encodingdata to generate second encoding data; and the means for generating isoperable to generate the transcoded data by combining the secondencoding data and the second parametric extension data.

The first encoding data may be encoded according to a first encodingstandard and may comprise sufficient information to allow independentdecoding based only on the first encoding data. The first parametricextension data may be enhancement data which may be used by a suitabledecoder to enhance the encoded signal. The first encoded data and theparametric extension data may be separately transcoded thereby allowingindividual optimization of the transcoding processes and thus improvedperformance and/or reduced complexity.

According to a different feature of the invention, the means fordetermining is operable to determine at least some of the secondparametric data by copying at least some data values of the firstparametric extension data. This may result in a low complexityimplementation and/or may increase the quality of the transcoded datastream. In particular, copying of at least some data values may preventany transcoding effects to be introduced to these data values.

According to a different feature of the invention, the means fordetermining comprises means for quantizing data values of the secondparametric extension data. The means for determining may re-quantizedata values as appropriate for the transcoded data stream. For example,the bit rate may be reduced by using a different (e.g. coarser)quantization for at least one data value of the second parametricextension data than is used for the first parametric extension data. There-quantization may be applied to data values which are copied from thefirst parametric extension data to the second parametric extension dataor may e.g. be applied to data values derived from the first parametricextension data, for example by interpolation.

According to a different feature of the invention, the means fordetermining comprises means for encoding data values of the secondparametric extension data. The means for determining may re-encode datavalues as appropriate for the transcoded data stream. The re-encodingmay be applied to data values which are copied from the first parametricextension data to the second parametric extension data or may e.g. beapplied to data values derived from the first parametric extension data,for example by interpolation.

According to a different feature of the invention, the means fordetermining is operable to determine at least some of the secondparametric data by interpolation between parametric extension datavalues of the first parametric extension data. This provides for a lowcomplexity means of determining second parametric extension datasuitable for the transcoded output stream. The term interpolation isherein used to include both interpolation and extrapolation.

According to a different feature of the invention, the means fordetermining comprises means for determining transient data of the firstparametric extension data and generating the second parametric extensiondata in response to the transient data .The determined transient datamay e.g. be a transient data value or may be a transient data position.This may provide improved quality of the transcoded data and mayspecifically result in a closer correspondence between the encodedsignal and the transcoded output stream. Transient data values may beincluded in the input data corresponding to sudden changes in theencoded signal. Specifically, the first parametric extension data maycomprise regular, substantially periodically occurring data values inaddition to transient values occurring at random intervals dependent onthe characteristics of the encoded signal. The transient values may e.g.used to calculate data values to be included in the second parametricextension data, for example by interpolation.

According to a different feature of the invention, the means fordetermining is operable to include at least one transient data parameterin the second parametric extension data. This allows the informationcomprised in a transient value to be retained in the transcoded dataresulting in improved quality and/or may provide for a low complexitytranscoding of parametric extension data comprising transient values.

According to a different feature of the invention, the means fordetermining comprises means for filtering the first parametric extensiondata prior to determining the second parametric extension data. This mayimprove the quality of the transcoded data and may specifically improvehigh frequency performance by compensating for low pass filteringassociated with interpolation operations.

According to a different feature of the invention, the input data andtranscoded data have non-synchronous frame structures and the means fordetermining the second parametric extension data is operable todetermine at least one data value associated with a frame of thetranscoded data in response to a first data value of a first frame ofthe first parametric extension data and a second data value of a secondframe of the first parametric extension data. This provides for a lowcomplexity, efficient and/or high quality transcoding between encodingformats having non-synchronous frame structures. The non-synchronousframe structures of the input data and the transcoded data mayspecifically have different frame lengths.

According to a different feature of the invention, the means fordetermining is operable determine the at least one data value byinterpolating between the first data value and the second data value.This provides for a low complexity means of determining secondparametric extension data suitable for the transcoded output stream. Theterm interpolation is herein used to include both interpolation andextrapolation.

According to a different feature of the invention, the first data valuecomprises a plurality of sub-values related to a first plurality offrequency sub-bands, the second data value comprises a plurality ofsub-values related to a second plurality of frequency sub-bands and themeans for determining is operable to determine the at least one datavalue to comprise a plurality of sub-values related to a third pluralityof frequency sub-bands. This provides for a low complexity means ofdetermining second parametric extension data suitable for the transcodedoutput stream.

According to a different feature of the invention, the first, second andthird plurality of sub-bands comprise the same number of frequencysub-bands. This provides for a low complexity means of determiningsecond parametric extension data suitable for the transcoded outputstream.

According to a different feature of the invention, the first pluralityof sub-bands comprise more frequency sub-bands than the second pluralityof sub-bands and third plurality of sub-bands comprise the same numberof frequency sub-bands as the first plurality of sub-bands. Thisprovides for a low complexity means of determining second parametricextension data suitable for the transcoded output stream.

The first and/or second parametric extension data may comprise SpectralBand Replication (SBR) parametric extension data and/or ParametricStereo (PS) parametric extension data.

According to a different feature of the invention, the parametricextension data is included in an auxiliary data section of thetranscoded bit stream. This may provide for backwards compatibility.Legacy decoders that are not capable of exploiting the parametricextension data may still decode the transcoded bit stream by ignoringthe auxiliary (or ancillary) data sections.

Preferably, the encoded signal is an audio signal.

According to a second aspect of the invention, there is provided amethod of transcoding comprising the steps of: receiving input datarepresenting an encoded signal and comprising first parametric extensiondata; determining second parametric extension data from the firstparametric extension data; and generating transcoded data including thesecond parametric extension data.

These and other aspects, features and advantages of the invention willbe apparent from and elucidated with reference to the embodiment(s)described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described, by way of exampleonly, with reference to the drawings, in which

FIG. 1 illustrates a block diagram of a transcoder in accordance with anembodiment of the invention;

FIG. 2 illustrates interpolation of data values of parametric extensiondata in accordance with an embodiment of the invention;

FIG. 3 illustrates interpolation of data values of parametric extensiondata in accordance with an embodiment of the invention;

FIG. 4 illustrates a principle diagram of a linear interpolator inaccordance with an embodiment of the invention;

FIG. 5 illustrates the frequency response of a filter of a linearinterpolator in accordance with an embodiment of the invention;

FIG. 6 illustrates an example time alignment between an mp3PRO inputstream and an aacPlus transcoded data stream;

FIG. 7 illustrates an example of timing of envelope data values of aninput data stream; and

FIG. 8 illustrates another example of timing of envelope data values ofan input data stream.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description focuses on embodiments of the inventionapplicable to an audio transcoder and in particular to an audiotranscoder for transcoding between input and output signals comprisingSpectral Band Replication (SBR) or Parametric Stereo (PS) parametricextension data. However, it will be appreciated that the invention isnot limited to these embodiments but may be applied to many othertranscoders and extension data.

FIG. 1 illustrates a block diagram of a transcoder 100 in accordancewith an embodiment of the invention.

In accordance with the embodiment, quality degradations associated withthe transcoding of parametric extension data may be mitigated orobviated by directly generating parametric extension data for outputtranscoded data from the parametric extension data of the input data. Inthe specific embodiment, the input data further comprises encoding datacorresponding to a signal encoded in accordance with a given encodingprotocol. In the embodiment, the parametric extension data isenhancement data which may be used by suitable encoders to improve thequality of the decoded signal. For example, the encoding data maycomprise a signal encoded in accordance with an audio encoding standardsuch as MP3 or AAC and the parametric extension data may comprise SBRand/or PS enhancement data.

Specifically, the transcoder 100 comprises a receiver 101 which receivesan input data stream comprising an encoded signal and parametricextension data. The receiver 101 is operable to de-multiplex the inputdata stream and to separate the input encoded data from the inputparametric extension data.

The receiver 101 is coupled to a decoder 103 which is fed the inputencoded data. In the embodiment, the decoder 103 decodes the inputencoded data in accordance with. the appropriate encoding standard andgenerates a pulse code modulated representation of the underlying audiosignal.

The decoder 103 is coupled to an encoder 105 which receives the pulsecode modulated data and encodes the signal to generate output encodeddata. The encoding protocol or standard of the encoder 105 is in theembodiment different than the encoding protocol of the input encodeddata. For example, the input signal may be encoded according to the MP3encoding standard and the encoder 105 may operate in accordance with theAAC standard.

In some embodiments, the same encoding protocol or standard may be usedwith different encoding parameters. For example, the encoder 105 may usethe same encoding standard but at a different bit rate than the decoder103.

The decoder 105 is coupled to an output processor 107 which is fed theoutput encoded data The output processor 107 includes the encoded datain a transcoded data stream.

The receiver 101 is furthermore coupled to an extension data processor109 which is fed the input parametric extension data. The extension dataprocessor 109 determines output parametric extension data from the inputparametric extension data. The output parametric extension data isgenerated to be compatible with and suitable as parametric extensiondata for the output encoded data.

The extension data processor 109 is coupled to the output processor 107which is fed the output parametric extension data. The output processor107 includes the output parametric extension data in the transcoded datastream.

Thus, in the described embodiment, an encoded signal is transcoded bysuing a conventional cascade of an encoder and a transcoder. Inaddition, parametric extension data of the input data is separatelyprocessed to generate suitable parametric extension data for the outputdata stream. Accordingly, the parametric extension data may be optimallyprocessed allowing increased quality of the transcoded data stream.Furthermore, a lower complexity transcoder may typically be implementedas the processing required for the generation of output parametricextension data is typically relatively simple and as the decoder andencoder can ignore the parametric extension data.

In a simple embodiment, where the frame lengths of the input data streamand the output data stream align, data may typically be copied directlyfrom the input parametric extension data to the output parametricextension data. For example, transcoding of an MP3 data stream at afirst bit rate comprising PS extension data to another MP3 data streamat a different bit rate may be achieved by transcoding the MP3 data bythe decoder and encoder and directly copying the PS extension data fromthe ancillary (or auxiliary) data sections of the input stream to theancillary (or auxiliary) data sections of the output data stream.

The extension data processor 109 may in some embodiments comprisefunctionality for re-encoding and/or re-quantizing data values of theoutput parametric extension data. For example, data values forInter-channel Intensity Differences may be quantized with a coarserquantization in order to reduce the data rate of the PS parametricextension data. Similarly a different encoding of the data values may beused to provide a desired characteristic such as for example a highererror resistance.

Typically, quantization and encoding of data values of the outputparametric extension data is particularly advantageous when the datavalues have been derived by calculations based on the data values of theinput parametric extension data.

It will be appreciated that in some embodiments, only the parametricextension data may be modified by the transcoder. For example, thetranscoding may extract parametric extension data from the ancillarydata sections of a bit stream, modify the parametric extension dataaccording to a given algorithm and re-insert the modified parametricextension data in the ancillary data sections.

In some embodiments, where the frame lengths of the input and outputdata streams do not align, data values of the output parametricextension data may be determined by interpolation (includingextrapolation) from the data values of the input parametric extensiondata This approach is suitable for most parametric extension dataparameters, as these tend to be slowly varying with time.

The following description will describe such an embodiment in moredetail with specific reference to Interchannel Intensity Difference datavalues but it will be appreciated the same principles may be applied tomany other parameters.

FIG. 2 illustrates interpolation of data values of parametric extensiondata in accordance with an embodiment of the invention.

In the example, the input parametric extension data comprises an IIDvalue for substantially regular time intervals of h_(a) (i.e. with ahop-size (or frame size) of h_(a). The IID values of the inputparametric extension data are indicated by crosses in FIG. 2, whichspecifically shows three IID values of the input parametric extensiondata at time intervals t₀, t₁ and t₂.

In the example, the output parametric extension data is required tocomprise IID values at substantially regular time intervals of h_(b)which are less than h_(a) (i.e. with a smaller hop-size (or frame size)of h_(b)). The IID values of the input parametric extension data areindicated by circles in FIG. 2, which specifically shows three IIDvalues of the output parametric extension data at time intervals t′₀,t′₁ and t′₂.

In the embodiment, the extension data processor 109 is operable togenerate the output IID values by interpolation. Specifically, asillustrated in FIG. 2, the output IID values are generated by a simplelinear interpolation between surrounding input IID values. Thus, theoutput IID values at t′₀ and t′₁ are generated from the input IID valuesat t₀ and t₁ and the output IID value at t′₂ is generated from the inputIID values at t₁ and t₂.

It will be appreciated that instead of linear interpolation other formsof interpolation or extrapolation may be used.

In some parametric audio coding schemes, additional parametric extensiondata parameters are generated at transient positions. For example PSparametric extension data typically comprises IID data values atsubstantially regular intervals as well as transient

IID values which are included when significant and fast transitions aredetected in the IID signal.

FIG. 3 illustrates interpolation of data values of parametric extensiondata in accordance with an embodiment of the invention. The example ofFIG. 3 corresponds to the example of FIG. 2 except that an additionaltransient IID value is included in the input parametric extension dataat time instant t_(T).

In order to retain the information contained in the IID value at t_(T),the extension data processor 109 is operable to generate an additionaltransient output IID value at t_(T). Specifically, the extension dataprocessor 109 directly copies the IID value at t_(T) to the secondparametric extension data.

In addition, the transient input IID value is used for interpolationwhen appropriate. Thus, as illustrated in FIG. 3, the output IID valueat t′₂ is now generated from the input IID values at t_(T) and t₂.

Linear interpolation results in a low pass filtering of the underlyingsignal such that quickly varying parameters are smoothed. For PS IIDparameters this will result in a narrowed stereo image. In order tocompensate for this effect, the IID parameters may be filtered beforethey are quantized.

A specific example wherein the PS extension data of an MP3(PRO)+PSbit-stream is translated to PS extension data of an aac(Plus)+PSbit-stream is described below. Typical hop-sizes at a sampling frequencyof 44.1 kHz for the PS parameters of these bit-streams is 1152 samples(2 granules or 1 frame of MP3 data) and 1024 samples (1 frame of AACdata) respectively.

The PS parameter translation using linear interpolation can beinterpreted as shown in FIG. 4. FIG. 4 illustrates a principle diagramof a linear interpolator 400.

The linear interpolator 401 comprises an upsampler 401 which upsamplesthe IID parameters by a factor of 9. The resulting signal isinterpolated (filtered) by means of a filter 403 having a triangularimpulse response. Finally the signal is down-sampled by a factor of 8down sampler 405.

FIG. 5 illustrates the frequency response of the filter of FIG. 4. Itcan clearly be seen that the triangular impulse response results in alow pass filtering.

In order to compensate for the smoothing caused by the linearinterpolation the IID values x(n) may be filtered by the following FIR(Finite Impulse Response) filter:

${y(n)} = {\sum\limits_{k = 0}^{K - 1}{a_{k}{x\left( {n - k} \right)}}}$

with α preferably being a linear phase impulse response, i.e.α_(k)=α_(k′−k−1). The final IID values that need to be re-quantized maybe delay compensated and calculated from:

${z(n)} = {c \cdot {y\left( {n - \frac{K - 1}{2}} \right)}}$

where c is a power-compensation constant that may be set such that thepower of z(n) is equal to that of x(n). In the example above,α=[−0.18,1,−0.18] can be used (K=3).

In a more advanced, and thus computationally more expensive embodiment,the actual up and down sampling illustrated in FIG. 4 may be performedand a non-triangular impulse response may be used to further improve there-sampling reconstruction.

In the following, a specific embodiment wherein the input data andtranscoded data have non-synchronous frame structures will be described.Specifically, a transcoder transcoding encoded data from a firstencoding protocol to a second encoding protocol having different framelengths will be described. The description will focus on an embodimentfor encoding an MP3 bitstream with SBR extension data (an mp3PRObitstream) into an AAC bitstream with SBR extension data (aacplusbitstream).

In the embodiment, it is assumed that the bandwidth of the MP3 encodingand the AAC encoding is substantially the same. Specifically, thetranscoder may determine the bandwidth of the MP3 encoding from theincoming bitstream and set the AAC encoder to have the same bandwidth.

The envelope and noise floor data values of SBR extension data haveconstraints related to when and how often they may occur in a frame. AnSBR decoder typically performs a sub-band analysis resulting in a numberof sub-band samples per core audio frame (e.g. N=18 for mp3PRO and N=32for aacplus). In order to handle time critical signals, the start borderof the first envelope and the stop border of the last envelope in aframe may in mp3PRO and aacPlus vary between [0, 6] (start border firstenvelope) and [N−1, N−1+6] (stop border last envelope) respectively.Consequently, if N is different for the input encoding protocol and theoutput encoding protocol, it is not always possible to simply copy theenvelope or noise floor data values from the input bitstream to thetranscoded bitstream.

FIG. 6 illustrates an example time alignment for envelope data valuesbetween an mp3PRO input stream and an aacPlus transcoded data stream. Inthe example, it can be seen that envelope data values from mp3PRO frame1,2 and 3 can be directly copied to corresponding frames of the aacPlusbit stream. However, for the envelope data value of MP3PRO frame 4, somedata will relate to one frame of the aacPlus bit stream whereas otherdata will relate to a different frame of the aacPlus bit stream.Although FIG. 6 specifically illustrates envelope data, it will beappreciated that the principle applies to other data values includingnoise floor values.

The envelope and noise floor data can simply be copied as long as thisdoes not violate the constraints of the aacPlus bit stream. However, ifsuch a copy is not possible, (parts of) envelope and noise floor datavalues must be combined into one envelope and noise floor data value.

FIG. 7 illustrates an example of a timing of envelope data values of aninput data stream. Specifically, FIG. 7 shows two envelope data valuesof the MP3PRO bitstream. The first envelope data value E₁ covers a timeinterval from t₀ to t₁ and the second envelope data value E₂ covers atime interval from t₁ to t₂. Each envelope data value E₁, E₂ comprises anumber of sub-values E_(1,1), E_(1,2), E_(1,3), E_(1,4), E_(2,1),E_(2,2), E_(2,3), E_(2,4) each of which in the particular example is ascale factor for a specific frequency band. Thus the number ofsub-values depends on the frequency resolution in the frame.

In the example of FIG. 7, the AACPlus transcoded data stream comprises aframe in a time interval t′₁-t′₀ overlapping the two time intervals ofthe MP3PRO data stream. Accordingly, a new envelope data value must becreated for the time interval t′₁-t′₀, and specifically the extensiondata processor 109 may generate an envelope data value comprising thescale factors determined by interpolation between the scale factors ofthe envelope data values E₁, E₂, e.g:

$E_{1,1}^{\prime} = {\frac{\left( {E_{1,1} \cdot \left( {t_{1} - t_{0}^{\prime}} \right)} \right) + \left( {E_{2,1} \cdot \left( {t_{1}^{\prime} - t_{1}} \right)} \right)}{t_{1}^{\prime} - t_{0}^{\prime}}.}$

Similar equations may be applied to generate the other scale factorvalues E′_(1,2): E′_(1,2) and E′_(1,2).

In SBR there are two possible frequency resolutions for envelope datavalues (the noise floors have only one possible frequency resolution).Accordingly, it can occur that (parts of) envelopes with differentfrequency resolutions need to be combined. In this case, the extensiondata processor 109 preferably generates envelope data values accordingto the highest frequency resolution. This is illustrated with theexample shown in FIG. 8.

FIG. 8 shows two envelope data values E₁, E₂ of the MP3PRO bitstream.The example is identical to that of FIG. 7 except that the secondenvelope data value E₂ comprises only two sub-values E_(2,1), E_(2,2).An envelope data value for the time interval t′₁-t′₀ of the AACPlustranscoded data stream may be determined by interpolation according toe.g:

$E_{1,1}^{\prime} = \frac{\left( {E_{1,1} \cdot \left( {t_{1} - t_{0}^{\prime}} \right)} \right) + \left( {\frac{E_{2,1}}{2} \cdot \left( {t_{1}^{\prime} - t_{1}} \right)} \right)}{t_{1}^{\prime} - t_{0}^{\prime}}$

Similar equations may be applied to generate the other scale factorvalues E′_(1,2): E′_(1,2) and E′_(1,2).

It will be appreciated that any suitable extension data may be used. Forexample, the parametric extension data may be spatial audio coding data.For example, rather than just including stereo image data, amulti-channel image may be parameterized an included in the extensiondata. In accordance with one such embodiment a stereo encoded signal maybe included as a backwards compatible component and the parametricextension data may include data that is able to convert these into amulti-channel representation (e.g. 2 channels to 5 channels). Of courseother scenarios are possible, e.g. 1 channel to 5 channels, 2 channelsto 4 channels etc.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. However,preferably, the invention is implemented as computer software running onone or more data processors and/or digital signal processors. Theelements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units and processors.

Although the present invention has been described in connection with thepreferred embodiment, it is not intended to be limited to the specificform set forth herein. Rather, the scope of the present invention islimited only by the accompanying claims. In the claims, the termcomprising does not exclude the presence of other elements or steps.Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by e.g. a single unit orprocessor. Additionally, although individual features may be included indifferent claims, these may possibly be advantageously combined, and theinclusion in different claims does not imply that a combination offeatures is no feasible and/or advantageous. In addition, singularreferences do not exclude a plurality. Thus references to “a”, “an”,“first”, “second” etc do not preclude a plurality.

1. A transcoder (100) comprising: means (101) for receiving input datarepresenting an encoded signal and comprising first parametric extensiondata; means (109) for determining second parametric extension data fromthe first parametric extension data; and means (107) for generatingtranscoded data including the second parametric extension data.
 2. Atranscoder as claimed in claim 1 wherein the input data furthercomprises first encoding data associated with the encoded signal and thetranscoder further comprises means(103, 105) for transcoding the firstencoding data to generate second encoding data; and the means forgenerating (107) is operable to generate the transcoded data bycombining the second encoding data and the second parametric extensiondata.
 3. A transcoder as claimed in claim 1 wherein the means fordetermining (109) is operable to determine at least some of the secondparametric data by copying at least some data values of the firstparametric extension data.
 4. A transcoder as claimed in claim 1 whereinthe means for determining (109) comprises means for quantizing datavalues of the second parametric extension data.
 5. A transcoder asclaimed in claim 1 wherein the means for determining (109) comprisesmeans for encoding data values of the second parametric extension data.6. A transcoder as claimed in claim 1 wherein the means for determining(109) is operable to determine at least some of the second parametricdata by interpolation between parametric extension data values of thefirst parametric extension data.
 7. A transcoder as claimed in claim 1wherein the means for determining (109) comprises means for identifyingtransient data of the first parametric extension data and for generatingthe second parametric extension data in response to the transient data.8. A transcoder as claimed in claim 7 wherein the means for determining(109) is operable to include at least one transient data parameter inthe second parametric extension data.
 9. A transcoder as claimed inclaim 1 the means for determining (109) comprises means for filteringthe first parametric extension data prior to determining the secondparametric extension data.
 10. A transcoder as claimed in claim 1wherein the input data and transcoded data have non-synchronous framestructures and the means for determining (109) the second parametricextension data is operable to determine at least one data valueassociated with a frame of the transcoded data in response to a firstdata value of a first frame of the first parametric extension data and asecond data value of a second frame of the first parametric extensiondata.
 11. A transcoder as claimed in claim 10 wherein the means fordetermining (109) is operable to determine the at least one data valueby interpolating between at least the first data value and the seconddata value.
 12. A transcoder as claimed in claim 10 wherein the firstdata value comprises a plurality of sub-values related to a firstplurality of frequency sub-bands, the second data value comprises aplurality of sub-values related to a second plurality of frequencysub-bands and the means for determining (109) is operable to determinethe at least one data value to comprise a plurality of sub-valuesrelated to a third plurality of frequency sub-bands.
 13. A transcoder asclaimed in claim 12 wherein the first, second and third plurality ofsub-bands comprise the same number of frequency sub-bands.
 14. Atranscoder as claimed in claim 12 wherein the first plurality ofsub-bands comprise more frequency sub-bands than the second plurality ofsub-bands and the third plurality of sub-bands comprise the same numberof frequency sub-bands as the first plurality of sub-bands.
 15. Atranscoder as claimed in claim 1 wherein the second parametric extensiondata is Spectral Band Replication (SBR) parametric extension data
 16. Atranscoder as claimed in claim 1 wherein the second parametric extensiondata is Parametric Stereo (PS) parametric extension data.
 17. Atranscoder as claimed in claim 1 wherein the encoded signal is an audiosignal.
 18. A method of transcoding comprising the steps of: receivinginput data representing an encoded signal and comprising firstparametric extension data; determining second parametric extension datafrom the first parametric extension data; and generating transcoded dataincluding the second parametric extension data.
 19. A computer programenabling the carrying out of a method according to claim
 18. 20. Arecord carrier comprising a computer program as claimed in claim 19.